1. Field of the Invention
This invention relates to diamond drag bits.
More particularly, this invention relates to diamond drag bits for soft sticky shale like earth formations.
2. Description of the Prior Art
Young earth formations that fail in the elastic mode or low end of the plastic range such as the Kaolinitic shales and high percent smectite shales typically found in tropical deposition zones are very difficult to drill.
Limited success has been achieved by soft formation roller cone bits and some fishtail type blade bits.
The roller cone bits easily become clogged with the clay like formation severely restricting the penetration rate of the bit.
Fishtail blade bits, prior to diamond bits, wear out very quickly requiring numerous bit changes resulting in prohibitive "tripping" cost wherein all of the drill pipe must be removed from the borehole prior to replacing the bit.
The performance of conventional diamond drag bits has been unsatisfactory due to the fact that the diamond cutters become clogged with the clay like shales thereby inhibiting bit penetration.
Diamond fishtail blade bits also have bottomhole cleaning problems that severely limit bit penetration rate.
A new cutting mechanism for failing rock is disclosed which overcomes the inadequacies of the prior art. The new cutting mechanism is neither a compression failure mode typical of rotary cone rock bits nor a pure shear failure mode typical of a diamond drag bit.